At Pass Labs we use Bergquist Silpads for thermally conductive
insulators on TO3P (Plastic packages) but not on the TO3
packages because they tend to shear and fail with the TO3.
This is not the case with the TO3P because of the smaller single
hole and the transistor's plastic insulation around the mounting
hole.

We've been happy with the Silicone pads in this application as
they are less messy and perform nearly as well as the
traditional Mica and thermal goo. As far as I can make out,
they have lower thermal impedance than claimed. However this
is assuming a certain degree of smoothness of the metal, which
is simply the flat surface of an anodized extrusion, and has a
somewhat polished look to the finish.

More recently with the First Watt product, the heat sink has
been bead blasted to make it more spiffy, and you'd think that
the slight dullness of the surface wouldn't make a difference.

In fact it made quite a bit of difference with the silicone pads -
apparently something like another 50% thermal resistance, and
I ended up using Mica and goo. The figures I get are 1.5 deg C
per watt with mica, and 2.1 with silicone. This is obtained by
running the devices at 20 watts and measuring the temperature
of the top of the case versus the heat sink right next to the
device.

If we take the ratings of the device itself, we see internally we
can expect .83 deg/watt, and so we see that Mica and goo
end up about 1/2 the total thermal resistance. If your surface
is not flat and polished, and/or you're pushing on the power
ratings of the device, consider good old messy mica and goop.

Interesting,
I never bothered measuring the difference but it seems obvious to me that the good old grease is doing a much better job especially with non flat surface. I really donít mind working with it. Whenever I open an amp and see the grease sticking out of the transistor surface (Peavey is a nice example) I always consider it as good craftsmanship. Perhaps thatís only my conservative opinion.
I recall reading something about thermal isolation that had the best of both worlds. Must have been an old Elector magazine.
It appeared to be some kind of silpad that had the ability to become softer when heat was applied. The principle was to burn in the transistor or mosfet or whatever had to be isolated and after a period of time turn on the screws to tighten the contact with the heatsink.

Hi Nelson,
I agree with your assessment as I've found the same thing. When repairing an amp with a rough heatsink I smooth it with 600 grit sandpaper and a milled block (flat). It does make a difference. Yes, you can see the differential temperature drop with a thermocouple thermometer.
Aluminum chips from the heatsink can pierce the sil pads , so watch for that. Same goes for solder balls.
-Chris

Nelson, which silpads were you using? the better ones are filled with boron nitride and are supposed to be equivalent to an alumina insulator, though they still may fall short of mica and goop on a bead-blasted surface. It may be Chomerics rather than Berquist that makes this particular silpad variation. A lot of times I put a thin film of goop on a silicone insulator anyway just to hold it in place while I'm trying to assemble things - best of both worlds...

There is a metal insulator. I saw it in Rockford Fosgate car amp. The insulator is a metal sheet, but somehow it doesnt conduct electricity. What is this called? Who sells them?
Also I wonder where to buy insulator sheet that is not small-cut. In size of A4 paper or bigger. Where to buy this un-cut insulator? The one I saw is like plastic, with yellow or orange color. Not mica, not that greyish-rubber like insulator.

The metal insulators are heavily anodized aluminum. I got
a whole bunch of them they are very old. Dynaco 400's
used this insulator plus the goo on both sides. A good deep
scratch would make them conductive.

I would like to buy some of those large sheet silicone insulators
too.

My English is not good. Is GOOP = silicon paste, white like your toothpaste? Anyone knows what's its chemical name?
Where to buy this GOOP and sheet insulators (the one that is like paper, not the stiff mica)